There are various systems for actuating a control surface of an aircraft. There are also systems with which it is possible to alleviate the load to which such a control surface is subjected.
Various load alleviation strategies are known. Active load alleviation may be effected for low-frequency side wind perturbations on a vertical tail plane (or VTP) of an aircraft. This load alleviation may be effected using measurements of these perturbations taken by a sideslip probe. Furthermore, passive alleviation of the wing loading due to gusts (gust load alleviation, GLA) can be implemented using gust perturbation measurements provided by an angle of attack probe (or AoA) or using vertical load factor measurements. In addition, maneuver load alleviation (or MLA) for the wing loading during a maneuver may be based on measurement of the vertical load factor, the roll rate or other appropriate parameters for detecting dynamic maneuvers.
With regard to the actuation of a control surface of an aircraft, use is made of systems based on position or systems based on force sensors.
Systems based on position generally comprise a slaving loop. The slaving loop includes a three-position hydraulic servo valve which controls a piston actuator. The piston actuator makes it possible to transmit a movement to the control surface by the intermediary of the piston. A flight control system of the aircraft sends a command that is representative of a target position of the piston. The actual position of the piston is measured and compared with the target position. The hydraulic servo valve is opened or closed in order to change the position of the piston as a function of the difference between the measured position of the piston and the target position. Thus, the actuator is able to obey the commands of the flight control system.
However, when said difference between the measured position of the piston and the target position is zero, the servo valve remains closed whatever the external loads. In order to protect the system from external loads when the servo valve is closed, it is necessary to provide pressure release valves in order to protect the actuator from overpressure. These valves make it possible to prevent overpressures in the actuator in the event of extreme loads. However, these valves do not make it possible to alleviate the load in the maneuver, turbulence or gust situations used for designing the plane. Therefore, this architecture is not entirely satisfactory.
Systems based on force sensors are normally used when the spoilers of the aircraft are pressed against the wings. In this case, the slaving loop of the system does not control the position of the piston of the actuator but implements specific force sensors, in particular load cells, which measure the loads experienced by the control surface.